This invention relates to a wet gas scrubber and more particularly, to a method and apparatus for the wet scrubbing of gaseous prilling-tower effluent.
Prilling towers are well known in the fertilizer industry for the production of particulate chemical and fertilizer products such as ammonium nitrate, urea and mixed fertilizers. A hot, concentrated solution or melt is sprayed downwardly against a rising stream of air. Sprays of the solution or melt solidify and particulates are collected from the bottom of the prilling tower. Hot air is exhausted from the top of the tower into the atmosphere. The spraying of solutions or melts results in formation of particulates which are mostly large enough to fall against the rising stream of air. However, particulates which are too small to fall against the rising air will be carried with the air from the tower into the atmosphere where they form a mist or fume which causes visible pollution.
It is desirable that this pollution be abated. To do this, the prilling-tower effluent must be cleaned before being released to the atmosphere. To accomplish this, scrubbing of gaseous prilling-tower effluent appears to be the most economical method which not only eliminates pollution but enables recovery of at least a portion of the chemical or fertilizer product which has been entrained in the exhaust air from the prilling tower.
Various proposals have been made whereby the desired cleaning of the exhausting gas may be activated.
In one proposed system, a modified cyclone-type scrubber is built directly onto the top of the tower. Water is used as the scrubbing liquid. This system has the disadvantages of being expensive, complex, and of difficult maintenance.
Another system that has been proposed contemplates returning the exhaust gas, through suitable ducting, to the bottom of the tower. A conventional wet gas scrubber is used to clean the recirculated gas. This procedure has the disadvantages again of cost and complication.
It has also been proposed to remove the particulate fume causing the pollution by various filtering procedures, either using dry or wet filtering media. When used dry, these media have the disadvantage that they eventually clog, and must be replaced. It is inherent in the filtering process that this must happen.
It has been proposed to use a wet filter system. Such a system is described by Akitsune and Takae, in Chemical Engineering Progress, Volume 69, at pages 72 to 78 (the issue of June, 1973). In the described system the exhausting air is first washed by liquid from a spray system at the top of a prilling tower, and then passed through the filters. The device is so arranged that the spray water and exhaust air are flowing counter-currently, but this inevitably means that the only liquid to reach the filters to wet them is that carried along by the air currents. Indeed Akitsune and Takae specifically advocate limiting the amount of water reaching the filter: too little causes it to run dry and clog, whereas too much causes an unacceptably high pressure drop. In a forced ventilation tower the velocity can be enough to carry over a significant amount of liquid, but in a natural draft tower the air velocities are much lower, and the liquid carry-over is substantially less. Thus although this system has the advantages of comparative simplicity, compact size and decreased load on the filters, this system still has the important disadvantage that, in time, the filters become clogged. Indeed the filter is almost operating as a mist eliminator, but, since the mist being eliminated contains entrained dust particles, it gets clogged. Such clogging causes increased pressure drop in the tower, which is a major operating disadvantage for both natural draft and forced ventilation towers. This system, therefore, is not suitable for a natural draft tower.